This invention relates to a method of preparing and executing a sequence program for a programmable sequence controller for such applications as machine tool control.
Sequence control executed by a sequence controller ranks along with feedback control as one of the two principal forms of automatic control, and it has recently found use in controlling a variety of industrial machines. In executing sequence control, a sequence program utilizing a prepared subroutine is written in accordance with the operating sequence of the machine which is to be controlled, and the program is loaded into the memory of a programmable sequence controller.
In sequence programming, the sequence can be expressed in terms of relay symbols, logical symbols or by Boolean algebra. The relay symbol method of representing the sequence is advantageous if consideration is given to the historical background of the old sequence controllers constructed by combining contact relays. However, the relay symbol system necessitates an extremely complicated process when applied to a machine tool of the type having an automatic tool changing mechanism. In a machine tool of this type a magazine is revolved to permit a desired tool to be extracted from a tool holder and then attached to a spindle head, or to permit a tool which is already mounted in a spindle to be extracted from the spindle and carried back to the original position where the tool is again accommodated in the tool holder. A complicated process of this type generally cannot be expressed in the form of a ladder diagram and must be programmed separately by means of assembly language or the like. Hence there is need for a method which can facilitate the writing of a sequence program for executing a complicated process.